Multielement radio-frequency antenna structure having helically coiled conductive elements

ABSTRACT

A multielement antenna for radio frequency transmission is provided having a plurality of helically formed, electrically conductive elements disposed in coaxial, longitudinally extending relationship that are structurally supported and protectively encased in a dielectric material comprising a fiber-glassreinforced synthetic resin matrix. Coupling of the helical elements through a coaxial cable to associated electronic equipment is effected by a helical conductor disposed in electromagnetically coupled relatioship to the conductive elements and which is also physically supported by a similar dielectric material reinforced with fiber glass.

United States Patent [191 Francis et al.

[ 1 3,7319% 51 June5,1973

MULTIELEMENT RADIO-FREQUENCY ANTENNA STRUCTURE HAVING HELICALLY COILED CONDUCTIVE ELEMENTS Inventors: Richard J. Francis; Clara A. Francis, both of 11855 Broad Street, Pataskala, Ohio 43062 Filed: July 26, 1971 Appl. No.: 165,510

US. Cl ..343/895, 343/873 Int. Cl ..H0lq l/36, HOlq 1/40 Field of Search ..343/895, 715, 873,

References Cited UNITED STATES PATENTS Buxton ..343/895 X Harris ..343/895 Harris "343/895 2,663,869 3,623,l l8 Monser ..343/895 X Primary ExaminerRudolph V. Rolinec Assistant ExaminerMarvin Nussbaum Attorney-William V. Miller, Eugene J. Mahoney and Robert E. Stefens ABSTRACT A multielement antenna for radio frequency transmission is provided having a plurality of helically formed, electrically conductive elements disposed in coaxial,

longitudinally extending relationship that are structurally supported and protectively encased in a dielectric material comprising a fiber-glass-reinforced synthetic resin matrix. Coupling of the helical. elements through a coaxial cable to associated electronic equipment is effected by a helical conductor disposed in electromagnetically coupled relatioship to the conductive elements and which is also physically sup ported by a similar dielectric material reinforced with fiber glass.

14 Claims, 10 Drawing Figures Adcock et al. ..343/895 X PAIENTED N 975 SHEET 2 BF 3 Y Q gwwyfl a PATENTEDJUN 5:915

SHEET 3 [1F 3 BACKGROUND OF THE INVENTION The antenna structures of this'invention are primarily adapted to mobile and fixed installations for both transmitting and receiving functions such as citizens-band operations in connection with automotive vehicles or marine communications although the antenna structures are adaptable to other frequency band allocations. In installations of this type, the antennas of prior art constructions comprise a single, electrically conductive element that is fed at one end and is effective as both a receiver and radiator of electromagnetic wave energy in the radio-frequency spectrum and is of a physical construction to accomodate the mechanical forces that may be applied as a consequence of vehicular movement. Antennas of prior art construction for mobile installations are most commonly an electrical quarter wave in length and metallic ranging in length from about 9 feet for 27 megahertz to about 6 inches for 470 megahertz. These antennas are usually vertically mounted and supported only at the bottom and are end-fed with the vehicle body which is formed from an electrically conductive material providing a ground plane essential to their operation. In two-way radio communications, the transceiver and antenna are interconnected by a coaxial cable with the center conductor connected with the antenna while the outer sheath is connected with the ground plane or vehicle body. This construction substantially limits the prior art antennas to an electrical quarter wave in length.

BRIEF DESCRIPTION OF THE INVENTION The antenna structure provided by this invention comprises, as the electrically conductive elements thereof, a multiplicity of helical elements. Interconnection with a coaxial cable is effected by a-coupling helix disposed in electromagnetically coupled relationship to the helical conductive elements. The lengths of the helices, diameter of the conductive elements, as well as diameter of the helix are selected to form a composite structure having a desired impedance at the design op,- erating frequency of a particular antenna structure having a length that is a half wave or more electrically but is substantially less in actual physical length. This multiple element design enables construction of an antenna having a standing wave ratio of the order of 1.1 to 1 which is comparatively low and a more advantageous impedance match with that of a connecting coaxial transmission cable. A desired nominal operating frequency within a frequency band throughout the 1-500 megahertz spectrum, or a specific operating frequency, is readily obtained for a particular antenna structure through appropriate selection of the component elements and physical configuration of each element while providing a wide band-width and maintaining a relatively high response and radiation characteristic for the entire band. A further advantage of the antennas of this invention is that they may be either mounted vertically or horizontally, since they do not require a ground plane, and provide either vertical or horizontal polarization. A structurally supporting body is formed for the selected elements from a dielectric material such as a synthetic resin reinforced with strands of fiber glass with the completed structure capable of accomodating the structural or mechanical forces encountered in a mobile vehicular installation.

These and other objects and advantages of this invention will be readily apparent from the following detailed description of embodiments thereof and the accompanying drawings.

DESCRIPTION OF DRAWING FIGURES FIG. 1 is a fragmentary elevational view, partly in section, of an antenna structure embodying this invention.

FIG. 2 is a transverse sectional view taken along line 2-2 of FIG. 1.

FIGS. 3 and 4 are graphic representations of the frequency response characteristics of the elements forming the antenna structure.

FIG. 5 is a fragmentary sectional view of a modified antenna structure.

FIG. 6 is a fragmentary elevational view, partly in section, of a further modified antenna structure.

FIG. 7 is a transverse sectional view taken along line 7-7 of FIG. 6.

FIG. 8 is a fragmentary elevational view, partly in section, of a further modified antenna structure.

FIG. 9 isa transverse sectional view taken along line 99 of FIG. 8. 1

FIG. 10 is a top plan view of a beam antenna utilizing the elements of this invention.

DETAILED DESCRIPTION OF THE INVENTION Having reference to FIGS. 1 and 2 of the drawings, an antenna structure embodying this invention is illustrated in detail. This antenna structure comprises a multiplicity of electrically conductive elements, indicated generally at 10, encased i a structurally supporting body 11 that includes a central in 12 and an outer coaxially formed sheath 13 although the core 12 and sheath 13 are preferably integrally formed in fabrication of a composite antenna. In this embodiment, the two elements 15 and 16 are adjacently disposed in helically coiled relationship forming amember which is effective, at thedesign radio frequencies, for radiation or reception of electromagnetic wave energy. The two elements l5 and 16 are helically coiled to define an elongated cylinder and each extends the full length of the antenna. p

The conductive elements 15 and 16 are preferably formed from small diameter copper wire, such as No. 26 and 28 A. W. 6., respectively, to obtain the desired electrical characteristics and, consequently, the elements will not be structurally self-supporting. An antenna structure of this invention is particularly adapted to utilization in the 1-500 megahertz frequency spectrum where a full-wave antenna will have a substantial length. In the case of equipment operating in the citi- Zens-band or marine frequency spectrum at the nominal operating frequency of 27 megahertz and 156-162 megahertz, respectively, a quarter-wave length will be of the order of 9 to 1.5 feet, respectively, and it will be readily seen that this length precludes reliance on the structural strength of the conductive elements for structural integrity of the antenna structure. Accordingly, a structurally supporting body 11 is provided to adequately maintain the several conductive elements in a specific configuration and permit vertical or horizontal mounting of the antenna. This body 11 is formed from a dielectric material which is a synthetic resin matrix having the necessary mechanical characteristics as to flexural strength and modulus for the specific design application to withstand the static and dynamic loads that may be encountered in vehicular installations and maintain the specific element configuration. Preferably, the synthetic resin matrix which may comprise a thermosetting polyester or epoxy, also includes strands of fiber glass 17 distributed throughout the body to enhance the mechanical properties of the antenna structure. These strands of fiber glass 17 are preferably oriented longitudinally of the antenna structure and are included in both the core 12 about which the helical elements 15 and 16 are wound and the outer sheath 13. The two helical elements 15 and 16 may be conveniently formed by simultaneously winding both elements on the core 12 which is performed in a preliminary step in the antenna fabrication process. Each turn of 3/16 inch. The diameter of the outer sheath 13 may be of the order of /16 inch and will become integral with the core 12 during the thermosetting step thereby providing a unitary structure. At least one of the helical elements, or 16,- may be provided with a dielectric sheath 18 to assure electrical insulation of the two elements throughout their length. This dielectric sheath 18 in the present embodiment comprises a suitable varnish; however, other well-known materials that do not provide electromagnetic shielding may be utilized. If desired, the dielectric sheath 18 may be omitted if the element spacing is otherwise maintained or both elements may be provided with a similar dielectric sheath.

Interconnection of the antenna structure with the radio components through a coaxial cable is effected by a helically formed, electricallyconductive element 19 having the terminal ends 19a and 19b adapted to be electrically connected to a coaxial cable (not shown) by the usual connector components (also not shown). In the FIG. 1 embodiment, the helical coupling element 19 is wound around the body 11 of the antenna structure in coaxial relationship to the helical elements 15 and 16 and is preferably axially positioned at the halfwave point although it may be positioned at the quarter-wave point. The element 19 may also be formed fromsmall diameter copperwire, such as No. 24 A. W. G., with the spacing of adjacent coils as well as the number of turns being determined to provide optimum coupling with a minimum standing wave ratio (S. W. R.) at the desired operating frequency. In the case of an antenna embodying this invention having a design operating frequency in the marine band of 156-162 MHz, the element 19 may comprise about 30 turns distributed over a length of about 4 inches.

Securance of the element 19 on the antenna structure may be readily accomplished by forming a layer of resin 20 with fiber glass strands 21 over the helical element 19. This layer of resin 20 will bond to the outer sheath 13 of the antenna forming a unitary structure and protectively fixing the helical element 19 in the desired position.

Through selection of conductive elements 15 and 16 of appropriate cross-sectional area and through proper spacing of the elements, an antenna structure may be constructed having a predetermined value antenna impedance. This impedance is determined by physical spacing and cross-sectional area of the conductors and is substantially independent of frequency of the electromagnetic wave energy. In the usual installation, the desired antenna impedance for proper matching is 5 20 as this is the impedance of the most commonly used commercially available coaxial transmission cable.

The antenna structure of this invention will preferably be a half-wave length or longer for the specificdesign frequency band. One of the parameters controlling the physical length of an end-fed electrical quarterwave antenna is the diameter of the conductor. For a given electrical full-wave, the physical length of the conductor decreases as the conductor diameter increases, but not as a straight line function. FIG-3 illustrates this condition. Curve M shows the response of a conductor of a given diameter, while curve N is the response of a conductor of another diameter. FIG. 3 shows that their resonant frequencies are at different frequency values in'the spectrum and illustrates how a multiplicity of conductors of dissimilar diameters broadens the effective band-width of an antenna.

FIG. 4 illustrates how conductors of different physical lengths have their maximum response at dissimilar frequencies. Curves P and S represent conductors of different physical lengths, and their resonant frequencies may be widely displaced.

A difference in length of the pairs of conductive elements 15 and 16 is obtained in the illustrated embodi ment where the two helical elements. 15 and 16 are seen to be of dissimilar diameters. This resultsfrom the two elements being wound with the same pitch with the same internal diameter and this causes the pitch diameter which determines the lineal dimension to be different. The larger diameter element of 15 and 16 will thus be longer.

FIG. 5 illustrates a modification of the antenna illustrated in FIGS. 1 and 2 with respect to the coupling element 19. In this modification, the coupling element 19 comprises a plurality electrically conductive elements, 22 and 23, that are helically wound on the basic antenna structure. These elements 22 and 23 are electrically insulated from each'other throughout their length, either by means of an insulative coating or by relatively spacing the elements within the dielectric layer 20, but are electrically connected together at each endfor connection to the coaxial cable (not shown). Better performance may be obtained through use of a multiple conductor coupling element. I

A further modified antenna is illustrated in FIGS. 6 and 7 which basically comprises two of the antenna elements previously described and disposed in slightly spaced, parallel relationship. Each of the two antenna elements designated generally by the numeral 25 is constructed as previously described having pairs of conductive elements 15 and 16 helically wound on a resin core 12 and encased within a resin sheath 13, both of which include strands of fiber glass to provide necessary strength. thetwo elements 25 are rigidly secured together at their mid-point to maintain the fixed relationship of the elements with a spacing of about three-eighths of an inch in the case of an antenna designed for operation at 27 MHz. This may be accomplished by forming a body of resin 26 around the elements and may include fiber glass 27 strands for additional rigidity. A helical conductive element 28 is wound around both elements 25 to provide coupling to an interconnecting coaxial cable (not shown). This coupling element may comprise about 30 turns of No.

24 A. W. G. copper wire distributed over a length of about four inches. The two elements 25 may be of slightly different physical lengths to provide resonance at two discrete wavelengths and thereby provide a broad band characteristic. In addition, a degree of tuning may be attained by relatively separating the elements 25 through flexing as is illustrated with respect to the one end of the antenna. The elements 25 are flexible to a degree and may be maintained in a divergent relationship by means of a spacer 29 formed from a dielectric material. This permits more closely matching the antenna impedance to that of the interconnecting coaxial cable (not shown).

The modified antenna structure shown in FIGS. 8 and 9 includes two antenna elements 30 constructed as illustrated in FIGS. 1 and 2 with each comprising a pair of conductive elements and 16 helically wound on a resin core 12 and encased within a resin sheath 13. Both the core 12 and sheath include strands of fiber glass to provide the necessary strength. In this modification, the two elements are spaced apart a distance of the order of 4 inches in the case of antenna designed for operation in the marine band of l56 162 MHz. Dielectric spacing elements 31 are utilized to secure and maintain the two elements 30 in the desired relationship. Coupling of the antenna to a coaxial feed cable is accomplished by a helical coupling element 32 supported in parallel relationship to one element 30 at about the midpoint thereof and comprising a relatively short length of the antenna element having connector leads 33 that are each electrically secured to both of the conductive elements 15 and 16 at relatively spaced points that are not at the ends of the coupling element. The coupling element 32 may be of a length of 9 inches for an antenna designed to operate in the marine band, l56-l62 MHz with the connector leads 33 connected at a spacing of about 5 inches. Dielectric connecting elements 34 may be utilized in securing the antenna element 30 and coupling element 32 together. The spacing of the two elements 30 may be varied to alter the antenna characteristics as may the relative spacing of the coupling element 32 and the one antenna element. In addition, the lengths of the two elements 30 may be dissimilar with the one disposed adjacent'the coupling element 32 being the longer. This provides a broad band antenna having two distinct resonant points within the band.

The coupling technique illustrated in FIGS. 8 and 9 may be utilized with a single antenna element 30. This construction will be readily apparent from the structure shown in these figures and it will not be necessary to further describe this construction.

A directional antenna structure utilizing either a director or reflector element 35 incombination with an active element 36 is illustrated in FIG. 10. The active element 36 will be of the construction described in conjunction with FIG. but the coupling may be either that shown in FIG. 1 or that shown in FIG. 8. The parasitic element 35 will also be of the same construction as the active element 36 but without the coupling element. The two elements 35 and 36 are supported on a rigid bar 37 in spaced parallel relationship with the length of the parasitic element 35 being either greater or less depending on whether operation is desired as a reflector or director.

Support of the antenna structures previously described may be either vertical or horizontal to obtain either vertical or horizontal polarization. Suitable support structures will be obvious and are therefore not described or illustrated. a

It will be readily apparent that a novel antenna structure is provided which provides a relatively standing wave ratio of the order of 1.1 to l and can be readily constructed for full wave operation. Optimum matching of impedance can be obtained and utilizing conductive elements of dissimilar diameter and length provides a broad band frequency response. The need for a ground plane is also eliminated thereby facilitating use of the antenna structure in marineinstallations where it is difficult to obtain satisfactory ground plane.

Having thus described this invention, what is claimed IS an antenna element including at least two elongated,

side relationship and formed into respective cylindrical helixes of the same pitch and internal diameter, each of said elements being electrically insulated from theother throughout the length thereof, said elements being of dissimilar lengths and cross-sectional area and relatively spaced to provide a desired antenna imped-' ance, a supporting body structure for said electrically conductive elements formed from a dielectric having a relatively low loss characteristic as to electromagnetic wave energy and physical strength characteristics to maintain the physical configuration of the conductive elements and assure structural integrity of the antenna structure, and a coupling element disposed in electromagnetically coupled relationship to said conductive elements and including at least one helically'wound, electrically conductive element. v

2. An r-f antenna structure according to claim 1 wherein said coupling element is coaxially disposed relative to said conductive elements.

3. An r-f. antenna structure according to claim 1 wherein said coupling element comprises at least two electrically conductive elements formed into respective cylindrical helixes with the two elements electrically insulated from each other throughout their length with terminal ends electrically interconnected.

4. An r-f antenna structure according to claim 1 wherein said coupling element is encased within a di-. electric material.

5. An r-f antenna structure according to claim 1 wherein said coupling element is disposed in spaced parallel relationship to said antenna element.

6. An r-f antenna structure according to claim 1 wherein said coupling element comprises a plurality of electrically conductive elements formed into a cylindrical helix and having two connector leads electrically connected thereto at longitudinally spaced points with the coupling element disposed parallel to said antenna element.

7. An r-f antenna structure according to claim 1 comprising a pair of said antenna elements disposed in laterally spaced electromagnetically coupled, parallel relationship.

8. An r-f antenna structure according to claim 7 wherein said coupling element is coaxially disposed relative to both of said antenna elements.

9. An r-f antenna structure according to claim 7 wherein said antenna elements are mechanically secured together in fixed relationship at their mid-point and are relatively flexible at their free end portions.

1. A radio-frequency antenna structure comprising- 10. An r-f antenna structure according to claim 7 wherein said antenna elements are of different lengths and the coupling element is disposed more closely adja- 'cent to the one of said antenna elements which is rela- 8 including a multiplicity of longitudinally extending strands of fiber glass.

12. An r-f antenna structure according to claim 11 wherein said synthetic resin is a thermosetting polyester.

13. An r-f antenna structure according to claim 11 wherein said synthetic resin is an epoxy.

- 14. An r-f antenna structure according to claim 11 in which said fiber glass strands are distributed through the core and sheath of said resin matrix. 

1. A radio-frequency antenna structure comprising an antenna element including at least two elongated, electrically-conductive elements disposed in side-by-side relationship and formed into respective cylindrical helixes of the same pitch and internal diameter, each of said elements being electrically insulated from the other throughout the length thereof, said elements being of dissimilar leNgths and cross-sectional area and relatively spaced to provide a desired antenna impedance, a supporting body structure for said electrically conductive elements formed from a dielectric having a relatively low loss characteristic as to electromagnetic wave energy and physical strength characteristics to maintain the physical configuration of the conductive elements and assure structural integrity of the antenna structure, and a coupling element disposed in electromagnetically coupled relationship to said conductive elements and including at least one helically wound, electrically conductive element.
 2. An r-f antenna structure according to claim 1 wherein said coupling element is coaxially disposed relative to said conductive elements.
 3. An r-f antenna structure according to claim 1 wherein said coupling element comprises at least two electrically conductive elements formed into respective cylindrical helixes with the two elements electrically insulated from each other throughout their length with terminal ends electrically interconnected.
 4. An r-f antenna structure according to claim 1 wherein said coupling element is encased within a dielectric material.
 5. An r-f antenna structure according to claim 1 wherein said coupling element is disposed in spaced parallel relationship to said antenna element.
 6. An r-f antenna structure according to claim 1 wherein said coupling element comprises a plurality of electrically conductive elements formed into a cylindrical helix and having two connector leads electrically connected thereto at longitudinally spaced points with the coupling element disposed parallel to said antenna element.
 7. An r-f antenna structure according to claim 1 comprising a pair of said antenna elements disposed in laterally spaced electromagnetically coupled, parallel relationship.
 8. An r-f antenna structure according to claim 7 wherein said coupling element is coaxially disposed relative to both of said antenna elements.
 9. An r-f antenna structure according to claim 7 wherein said antenna elements are mechanically secured together in fixed relationship at their mid-point and are relatively flexible at their free end portions.
 10. An r-f antenna structure according to claim 7 wherein said antenna elements are of different lengths and the coupling element is disposed more closely adjacent to the one of said antenna elements which is relatively longer than the other.
 11. An r-f antenna structure according to claim 1 wherein said supporting structure is a fiber glass reinforced, synthetic resin matrix having a central core around which said electrically conductive elements are helically wound and an outer sheath in which said core and conductive elements are encased, said structure including a multiplicity of longitudinally extending strands of fiber glass.
 12. An r-f antenna structure according to claim 11 wherein said synthetic resin is a thermosetting polyester.
 13. An r-f antenna structure according to claim 11 wherein said synthetic resin is an epoxy.
 14. An r-f antenna structure according to claim 11 in which said fiber glass strands are distributed through the core and sheath of said resin matrix. 